c) Acoustic impedance

• acoustic impedance

• a measure of how much a medium resists the passing of an ultrasound wave through it

• the formula for acoustic impedance

• Z = pc

  • Z = acoustic impedance (kgm^-2s^-1)

  • ρ = density of the medium (kgm^-3)

  • c = speed of ultrasound (ms^-1)

• the ratio of the reflected intensity to the initial intensity of an ultrasound wave

• for normal incidence

• Ir/I0 = (Z2 - Z1)2 / (Z2 + Z1)2

• coupling gel

• a gel with an acoustic impedance

• similar to that of skin

• smeared on the skin and the transducer

• before an ultrasound scan

• the need for coupling gel

• the transducer is placed on the skin

• there are air gaps between the skin and the transducer

• air and skin have a different acoustic impedance

• most ultrasound is reflected at the boundary

• the coupling gel fills air gaps

• most ultrasound is transmitted through the boundary

• impedance matching

• when two media have similar acoustic impedances